Sputtering target assembly and method of making same

ABSTRACT

A method for producing a sputtering target assembly bonded to a backing plate. The method includes bonding a target to a high strength backing plate and further creating a vacuum seal between the target and the backing plate using friction stir welding processes.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. patent applicationSer. No. 12/310,857 filed Mar. 10, 2009, which represents the U.S.National Phase Application of PCT Application No. PCT/US2007/017598filed Aug. 8, 2007, which claims priority under 35 U.S.C. §119(e) toU.S. Provisional Patent Application Ser. No. 60/844,028 filed Sep. 12,2006.

FIELD OF THE INVENTION

The present invention relates to methods for preparing sputtertarget/backing plate assemblies, and to the target/backing plateassemblies prepared by these methods. More particularly, the inventionrelates to methods for bonding sputter targets to an associated backingplate using friction stir welding techniques.

BACKGROUND OF THE INVENTION

Cathodic sputtering is widely used for depositing thin layers or filmsof materials from sputter targets onto desired substrates. Basically, acathode assembly including the sputter target is placed together with ananode in a chamber filled with inert gas, preferably argon. The desiredsubstrate is positioned in the chamber near the anode with a receivingsurface oriented normal to a path between the cathode assembly and theanode. A high voltage electric field is applied across the cathodeassembly and the anode.

Electrons ejected from the cathode assembly ionize the inert gas. Theelectrical field then propels positively charged ions of the inert gasagainst a sputtering surface of the sputter target. Material dislodgedfrom the sputter target by the ion bombardment traverses the chamber anddeposits to form the thin layer or film on the receiving surface of thesubstrate.

In order to achieve good thermal and electrical contact between thetarget and the backing plate, these members are commonly attached toeach other by way of soldering, brazing, diffusion bonding, clamping,epoxy cements, or with interlocking annular members. High purity metaland metal alloy sputter targets have also been mechanically bonded tobacking plates, for example as disclosed in U.S. Pat. Nos. 6,749,103 B1and 6,725,522 B1.

The relatively high yield strength coefficient associated with copperand copper alloy or aluminum alloy backing plates, compounded by the useof higher levels of sputtering power required to energize largersputtering targets, has increased the material stresses imposed on thebonds that join the sputter targets to the backing plates. Under suchsputtering conditions, the sputtering target assemblies tend to deflector separate upon exposure to the changing temperatures that aretypically encountered during sputtering. To a certain extent, softsolders have accommodated stresses exerted on the sputter target/backingplate assemblies as the assemblies are heated during the sputteringprocess and subsequently cooled. When low strength backing plates havebeen employed in the target/backing plate assemblies, the assembliestend to deflect significantly during sputtering causing severalundesirable effects, such as additional particle generation, decreaseduniformity, etc.

Accordingly, there remains a need in the art for a method for bondingsputtering target materials to an alloy backing plate wherein theassembly exhibits significant strength and yield strength. Althoughsputtering target assemblies may be made using high strength backingplate materials such as Al—SiC composite material (U.S. Pat. No.6,183,686) or Al alloy 7075, such materials are often impossible to weldat circumferential surfaces due to their chemical composition and poorweldability. In some other cases, there is a need to join an Al alloytarget to a Cu alloy backing plate, which is difficult without applyinghigh temperature bonding processes, such as a diffusion bonding process.As is known in the art, such high temperature processes may result inundesirable Al alloy target recrystallization or weak bond strength dueto formation of brittle intermetallic compounds between the metals (inthis case Cu and Al).

SUMMARY OF THE INVENTION

In accordance with the invention, a target and backing plate arefriction stir welded (f.s.w.) together to provide a high strength bond.In one aspect of the invention, the target and backing plate are bondedusing low temperature mechanical interlocking techniques such as thoseset forth in U.S. Pat. Nos. 6,749,103 and 6,725,522. The disclosures ofthese two patents are incorporated by reference herein. The jointexisting around the circumferentially extending target/backing plateinterface may then be bonded via friction stir welding.

In another aspect of the invention, the target plate/backing plate maybe joined along the plate/plate interfacial surface by friction stirwelding. Stated differently, the target and backing plate are joined byf.s.w. along the major joint plate interface that is circumscribed bythe perimeter of the assembly.

In both cases, the f.s.w. bonding enables the assembly to withstand thestress and degradation typically encountered during sputtering.

In another aspect, the backing plate comprises alloys with high yieldstrength such as Al 7000 series, Al 8000 series, and any other alloybased on Al, or Cu and it alloys, such as Cu-1% Cr, Cu-1% Zr, Cu—Be orCu—Zn, Cu—Ni and similar alloys. Any backing plate material may be usedhowever.

These and other features and advantages of this invention are describedin, or are apparent from, the following detailed description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side elevational view of a sputtertarget/backing plate assembly fabricated according to the presentinvention;

FIG. 2 is a diagrammatic plan view of the target and backing plateassembly of FIG. 1 showing the circumferential joint area that is to bebonded by friction stir welding; and

FIG. 3 is a diagrammatic top view showing a spiral f.s.w. jointpattern-formed along the backing plate surface to be bonded.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagrammatic side view of a preferred sputtering targetassembly in accordance with the inventive methods herein disclosed. Thesputtering target assembly includes a sputter target 100, superposedatop a heat conductive backing plate 110. As envisioned, the target 100and backing plate 110 are bonded by any method including the lowtemperature mechanical interlocking techniques set forth in either U.S.Pat. No. 6,749,103 or 6,725,522. As shown, the butt like joint formedalong the circumferential interface 120 between the components is bonded130 by f.s.w. through the backing plate. Cooling water (not shown) maybe circulated in contact with the backing plate so as to dissipate heatresulting from the sputtering operation. To prevent corrosion from thecooling water, an anticorrosive layer (not shown) may be disposed on therear surface of the backing plate.

One problem encountered in producing mechanically joined bondedsputtering target assemblies lies with the difficulty in producing asputtering target assembly having an hermetic seal at the targetdiameter. In the past, in order to achieve this hermetic seal, thetarget and backing plate were welded together by means of usual weldingmethods, such as electron beam welding, resistance welding or similar.However, some particularly desirable alloys were not suitable forwelding methods involve melting of metals. An example of a sputteringtarget assembly system particularly vulnerable to this problem is anassembly with a series 7000 Al alloy backing plate, which is prone tohot cracking upon exposure to elevated temperatures encountered duringwelding and, for example, a Cu-40% Zn alloy, which is prone to Znevaporation during electron beam or other welding techniques undervacuum.

A solder bonded target assembly often combines materials having suchundesirable welding properties. Consequently, alternative methods forforming an hermetic seal at the target/backing plate circumference mustbe used. One of the methods that may be used is HIPing, however, thismethod requires substantial additional operation and elevatedtemperature, which may damage the target material microstructure due toannealing process.

Turning back to FIG. 1 and also now considering FIG. 2, an embodiment ofthe invention is depicted wherein the circumferential joint 130 betweentarget and backing plate is f.s.w. bonded. As per FIG. 1, the f.s.w.tool 140, as shown, is applied against the assembly from the backingplate side.

As is known in the art, frictional stir welding (f.s.w.) relies upon thehigh rotational speed of the tool and resulting heat created fromcontact to crush, mix, and forge a bond between the metal surfaces to bejoined. Pin type f.s.w. tools are commercially available. F.s.w.techniques are explained in U.S. Pat. Nos. 5,460,317 and 6,758,382, bothincorporated by reference herein.

FIG. 3 is a diagrammatic top view of an assembly wherein the majortarget/backing plate 110/110 interfacial surfaces are f.s.w. bonded.That is, these major interfacial surfaces are circumscribed by theperimeter of the assembly and are bonded by f.s.w. Here, the f.s.w.pattern 150 can be viewed, in phantom looking through the top platesurface. The spiral pattern 150 shown depicts the weld zone made by aretractable pin type f.s.w. tool 140 or the like impressed upon theassembly from the bottom side of the backing plate 110. The pin maypenetrate through the backing plate 110 to a point proximate thetarget/backing plate 110/110 interface or it may terminate in thebacking plate 110 adjacent such interface. (In some cases, it may bepossible to penetrate slightly into the target 100). In this way, thetarget 100 and backing plate 110 are joined by the patterned weld zone.Although, a spiral type pattern 150 is shown in this drawing, other weldpatterns such as concentric circles, concentric triangles, squares, dots(ordered or random) etc. can be envisioned. The weld zone area made bythe f.s.w. tool 140 must be sufficient when compared to the interfacialarea to be joined, to ensure that a strong, durable f.s.w. joint beprepared.

Turning back to the embodiment shown in FIGS. 1 and 2, it is to beremembered that the major target plate/backing plate 100/110 bonding inthe area 160 circumscribed by the perimeter of the assembly may beeffected by a variety of different techniques. Most preferably, thesesurfaces are bonding by the low temperature mechanical interlockingtechniques previously described wherein one of the interfacial surfacesto be bonded is provided with ridges or other projecting surfaces whichpenetrate into and bend around the engaging mating surfaces provided onthe opposite, mating surface. The structural arrangement and methoddescribed in U.S. Pat. No. 6,749,103, FIG. 4, are particularlynoteworthy in this regard. Conversely, instead of such low temperaturemechanical interlocking techniques, bonding can be achieved via solderbonding, diffusion bonding, explosion bonding, HIPing, hot press, TIGwelding, and other conventional techniques.

To illustrate one exemplary embodiment of the invention, described belowis an example of producing a bonded Al-0.5% Cu sputtering targetassembly comprising an Al 7075 alloy backing plate. Although thisexample is of an Al 7075 alloy it is apparent that the process may beusefully applied to backing plates of other materials such as alloys ofCu, Ni, other Al alloys, etc. Similarly, various target materials may beemployed. In this example, a sputtering target assembly is made using abacking plate composed of Al 7075. In another example, a sputteringtarget assembly is made using a backing plate composed of Al 6061.Advantageously, a mechanical bonding process is used, including bondingan Al-0.5% Cu target to the backing plate to allow control over thedeflection of the assembly.

EXAMPLE 1

A backing plate 110 material is prepared for bonding by machiningcontinuous channels, for example an “M” pattern, on the backing plate110 surface to be bonded. Once the interfacial surfaces 120 are preparedfor bonding, the target 120 and backing plate 110 are assembled bymating the described prepared surfaces 120 and bonding them together bypressing the assembly at room temperature. See U.S. Pat. No. 6,749,103.After mechanical bonding, the assembly is friction stir welded aroundthe target circumference 160 to create a vacuum hermetic seal around thetarget 100. The f.s.w. can be both done from the target 100 face as wellas from the backing plate 110 side.

EXAMPLE 2

A backing plate 100 material is prepared for bonding by grit blastingand chemical cleaning of the surfaces 120 to be bonded. In order toimprove wetting of the bonding surface 130 and to improve adhesion ofthe solder material, both surfaces may be covered by an appropriatewetting or metal layer, preferably nickel, by an appropriate technique,preferably sputtering. Once the interfacial surfaces 120 are preparedfor bonding, the target 100 and backing plate 110 are assembled bymating the described prepared surfaces 120 and solder bonding themtogether by pressing the assembly at a temperature sufficient to meltthe solder material, for example 200° C. in the case of indium. Afterthe soldering process is completed, and has joined the major plate likeinterfacial surfaces 120 the assembly is f.s.w. bonded 130 around thecircumference 160 between the target 100 and the backing plate 110 tocreate an hermetic seal layer between the target 100 and backing plate110.

EXAMPLE 3

A backing plate 110 material is prepared for f.s.w. bonding by machiningthe surfaces 120 to be bonded. Once the interfacial surfaces 120 areprepared for bonding, the target 100 and backing plate 110 are assembledby mating the described prepared surfaces and f.s.w. bonding themtogether along the major plate/plate interface 120 such as in the FIG. 3embodiment by an f.s.w. pattern covering all of the bonded surfaces,applied for example in a spiral f.s.w. pattern 150, or any otherapplicable f.s.w. pattern, between the target 100 and backing plate 110to create the desired f.s.w. bond 130 between target 100 and backingplate 110.

While the methods described herein and the sputter target/backing plate100/110 assemblies produced in accordance with these methods constitutepreferred embodiments of the invention, it is to be understood that theinvention is not limited to these precise methods and sputtertarget/backing plate 100/110 assembly structures, and that changes maybe made in either without departing from the scope of the invention,which is defined in the following claims.

1. A method of producing a sputter target/backing plate assemblycomprising the steps of: a) providing a backing plate having a firstmating surface and an opposed bottom surface; b) providing a sputtertarget having a second mating surface; c) friction stir weld (fsw)bonding said backing plate and said sputter target together to form anassembly, said friction stir weld bonding being characterized bypenetration of a pin type fsw tool through said bottom surface of saidbacking plate.